The invention relates to improvements in devices for the application of preselected torque to rotary parts, particularly to screws, nuts, bolts and other externally or internally threaded fasteners. For example, devices of the type to which the present invention pertains can be utilized with advantage for the application of a preselected torque to tightening screws in hose clamps and the like.
It is already known to provide a device which can be applied to the head of a screw or another threaded fastener and can be rotated by a wrench, by a screw driver or by another suitable tool in order to rotate the fastener until the fastener begins to offer a preselected resistance to further rotation. Reference may be had, for example, to commonly owned U.S. Pat. No. 5,176,050 granted Jan. 5, 1993 to Heinz Sauer for "Tool for the application of predetermined torque to bolts, nuts and the like." The device of Sauer comprises a first section which can be permanently affixed to the head of a screw and a second section which becomes separated from the first section in response to the application of a preselected torque by a wrench or by another suitable tool, namely when the screw begins to offer a predetermined resistance to further rotation. Separation of the second portion from the first portion of the device which is disclosed by Sauer et al. constitutes evidence that the screw was driven home in response to the application of a preselected torque, namely a torque which was necessary to separate the second section from the first section. However, an unauthorized person is not prevented from rotating the screw in a direction to drive the screw deeper into one or more objects or to loosen the screw. In other words, the device of Sauer can furnish evidence that a preselected force was indeed applied to drive the nut home but such device cannot indicate whether or not the screw has been tampered with subsequent to separation of the second section from the first section. As a rule, the head of the screw has a polygonal or otherwise configurated internal surface which can be engaged by the working end of a suitable tool (such as a screw driver or a tool having a polygonal working end insertable into a complementary socket in the head of a screw) to be rotated subsequent to separation of the second section from the first section of the device which is disclosed by Sauer et al. Furthermore, and since the first section is non-rotatably secured to the head of the screw, and if such first section is provided with a polygonal, oval or other non-circular external surface, the screw can be driven deeper into one or more objects or can be loosened by a tool which is used to engage the external surface and to thereupon rotate the first section of the device subsequent to separation of the second section.
If the device of Sauer is utilized in a motor vehicle and a screw or another rotary part is either loosened or driven home with an excessive force, the maker of the vehicle or a person in charge of repairing the vehicle can prove that, by having utilized the device of Sauer, the rotary part was driven home in response to the application of a prescribed (preselected) torque. However, this does not suffice to relieve the maker or the repairman of responsibility for loosening or excessive application of the rotary part subsequent to the application of a preselected torque with the device of Sauer. This can result in a controversy and the maker of the vehicle and/or the person in charge of repairing the vehicle might be liable for damages to and for repair of the vehicle under an existing warranty or at such person's own expense.
German Auslegeschrift No. 1 134 252 of Heil (published Aug. 2, 1962) discloses a screw with two coaxial heads which are disposed end-to-end and one of which is of one piece with the externally threaded shank of the screw. The other head becomes separated from the one head in response to the application of a preselected torque by a screw driver or an analogous tool. The one head is confined in a socket which is installed in one of the objects to be connected to each other by the shank of the properly applied screw, and the socket is thereupon filled, e.g., with an insert consisting of lead, when the separation of the other head is completed. Absence of the insert or visible damage to the insert constitutes evidence that the device has been tampered with subsequent to separation of the other head from the one head and subsequent to filling of the socket with an insert of lead. The one head is devoid of slots or like configurations which would permit engagement by a screw driver, by a wrench or by any other suitable tool for the purpose of loosening or withdrawing the screw subsequent to removal of the insert from its socket. Therefore, the inventor proposes to form slots in the one head after removal of the insert so that the thus shaped one head can be engaged by a suitable implement and loosened or withdrawn from the part or parts which receive the shank of the screw. A drawback of the device of Heil is that the twin-headed screw is not a standard screw, that the device must employ an accurately machined or otherwise finished socket, that the application of an insert of lead takes up much time, that the removal of the insert takes up much time, and that the one head must be treated subsequent to removal of the insert in order to permit engagement of the one head by a suitable tool which is to loosen or remove the screw.
German Utility Model No. 90 11 571.6 of Petri (published Feb. 14, 1991) discloses a device wherein one end face of a headless screw is provided with a polygonal recess. The one end face is of one piece with one axial end of a tubular neck section which, in turn, is of one piece with a hexagonal second section. The aligned axial passages of the two sections contain a mass of compacted steel wool, felt, foamed plastic or the like in order to prevent access to the recess of the screw while the latter is still of one piece with the two sections. These sections can be separated from the screw at the end face in response to the application of a preselected torque, and this immediately affords access to the recess in the end face of the screw, i.e., the screw can be tampered with subsequent to the application of a preselected torque as evidenced by separation of the two sections from the end face of the screw. Thus, the device of Petri also fails to prevent tampering with a rotary part subsequent to the application of a preselected torque.
U.S. Pat. No. 4,037,515 granted Jul. 26, 1977 to Kesselman discloses a tamper resistant fastener wherein a stud engaging portion of steel can be applied over an externally threaded stud and becomes separated from a hexagonal wrench engaging portion in response to the application of a preselected torque to the hexagonal portion. The stud engaging portion is surrounded by a slip ring which is freely rotatable thereon and has a cylindrical external surface so that it cannot be rotated by a standard wrench or by a like implement. The narrow clearance between the slip ring and the stud can be overlapped by a flange of the slip ring to thus prevent the insertion of a jamming tool between the split ring and the stud. A drawback of the patented fastener is that the stud cannot be readily rotated once the hexagonal portion is broken off the stud engaging portion, and that the slip ring must be machined or otherwise treated and installed with a high degree of accuracy.